Sometimes writers use plot devices without realizing they're using them. This can produce results good and bad. Most writers are aware of the Deus ex machina plot device (or should be if you read this blog, because I covered it in this post).
As a writer, you should be aware of techniques like these. They can help or hamstring your plot. They can save or sink the story. If well-used, you look like a genius. If poorly applied, you look like a hack.
So before you get out your Chekov's gun to start blasting away at those alien space bats, make sure you know exactly what it is you're shooting.